Tranny Engagement Problems

[73c20]

hey, i have a 1973 C20 with the 350/th400 combo. when i havent driven the truck for a while, the tranny tends to not want to engage when i shift into any gear. but when i add a bit of tranny fluid, it works fine. i added a floorshift kit, and that was around the time it started. i know its nothing to do with the new linkage. any thoughts?

[Blazin]

If you have to add fluid then thwere must be a leak.